This is the opening paragraph of a blog post for a small business. It might get read by a couple thousand people. The writer is stepping out of a challenging job and wishing good luck to his successor. The tone is knowledgeable and sincere, but decidedly informal (the post contains several decent puns and ends with a very bad pun).
It’s been 90 years since horse-drawn wagons were an ordinary sight on the streets of Cleveland, but the expression “handing off the reins” remains common. The modern equivalent is “your turn to drive," with the crucial difference that the wagon’s horsepower was alive, with one or more minds of its own.
There are several phrases in the paragraph that I see as less than optimal, but a couple of people…
have problems with the use of horsepower as singular noun – and a singular object that could have “one or more minds”. That usage doesn’t bother me at all in this context. Should it?
Given the tone of that writing style, it all looks fine to me. The horsepower thing is a synecdoche and is fine as is. Although “horsepower” is a unit, it can also be used as a synonym for “power,” which is a mass noun, so OK to use the singular “was.” That a singular object could have one or more minds is a deliberate twist to the writing and IMHO is also fine.
You could rewrite it for nitpickers and pedants but then it would lose the distinct voice of the writer.